The wind turbine market is changing fast nowadays. There is a continuing demand for larger wind turbines being able to generate a higher number of megawatts of electricity, also referred to as multi-megawatt wind turbines. At the same time the requirements for reduction of size and weight of the turbines and their components become more and more important.
In wind turbines, typically a wind turbine rotor drives a low speed shaft of a gear transmission unit or gearbox, which transforms torque and speed of the rotor to the required torque and speed of an electrical generator. The increasing demand for multi-megawatt wind turbines puts a challenging pressure on new designs of components such as gearboxes for such wind turbines. This is because weight and cost of the wind turbine are to be kept as low as possible or at least within acceptable ranges, while at the same time it has to be made sure that the components can withstand high rotor loads being generated during operation of the wind turbine. One way of reducing the weight of components for the gearbox is by using less material and thus by making them thinner. This may give some kind of flexibility to these components. This flexibility may increase the possibility of deformation during operation of the wind turbine. This is because, during such operation, wind turbines and especially multi-megawatt wind turbines, create high dynamic forces to and speed variations in the gearbox. Because of that, loads and speeds during operation of the gearbox can differ from the design loads and speeds, i.e. from the predicted loads and speeds during design of the gearbox, and even reverse loads can occur. Because of these high specific forces and loads in gearboxes for wind turbines and the requirements with respect to weight of the gearbox as described above, when making the design of the gearbox, the possibility of potentially large deformations of, for example, the planet gears, should be taken into account.
In prior art designs, roller bearings have mostly been for supporting the planet gears. However, currently also the use of plain bearings as planet bearings has been studied.
In EP 2 383 480 a planetary gear unit for a wind turbine is described. The planetary gear unit comprises a sun gear, a ring gear and a planet carrier wherein a plurality of planet gears are bearing mounted on planet shafts. The planetary gear unit furthermore comprises a plurality of radial and axial plain bearings for supporting the planet gears. The radial plain bearings each comprise a bushing formed of a plain bearing material which is either mounted as an inner ring to the planet shaft or as an outer ring in a bore of a planet gear, whereby a corresponding outer ring or inner ring is formed respectively by the bore of the planet gears or by the planet shaft. The axial plain bearings each comprise a first bearing element formed of a plain bearing material which is provided on a contact surface between a planet carrier wall and a front side of a planet gear, either on the planet carrier wall or on the front side of the planet gear, and whereby a corresponding second bearing element is formed respectively by the front side of the planet gear or by the planet carrier wall.
The plain bearing arrangement described in EP 2 383 480 is limited to be used with a cage type planet carrier in which the planet gears are placed between two walls of the planet carrier and where these walls support the planet shafts on either side of the planet gears.
Furthermore, when using plain bearings for supporting planet gears, deformations of the planet gears as described above may be disadvantageous for these bearing arrangements. The plain bearing may wear-out locally, which may reduce its effectiveness and may even lead to failure of the bearing.